The present invention relates to a method of and arrangement for applying a surface pressure to workpieces which are driven by a pressing band, for example, continuously movable material webs, also in form of laminates or the like.
Methods and arrangements of the above-mentioned general type are known in the art. One of such methods and arrangements is disclosed in the German document DE-OS 24 21 296. In this reference the surface pressure is applied in an action zone which it limited at its one side by the pressing band, under the action of the pressure of a gaseous or liquid pressure medium acting in the action zone. A pressure chamber can be limited by a peripherally closed substantially rectangular seal over the periphery of the pressure chamber. At its one side, the pressure chamber is limited by a rotatable pressing band, while at its rear side it is limited by a pressing plate which simultaneously forms a heating plate for heating the pressure medium in the action zone. Band presses which are provided with such pressure chambers, for example, double-band presses, are also used for heating and/or cooling of the movable workpieces.
When the fluid pressure media used here are for example air or oil, controllability and sensitivity of adjustment are especially suitable. However, a problem remains in that the pressure gas layer which is located in the action zone between the rear side of the hot plate and the endless pressing band in the pressure chamber acts in an insulating manner with respect to the heat transfer. It has been proposed in the above-mentioned German document to replace strongly insulating gaseous pressure media with a liquid pressure medium, especially oil or liquid metal. While liquid metals for this goal and in this area of application technically cannot be considered as controllable, the use of oil does not bring radical improvements since the heat conductivity of oil is maximum three times the heat conductivity of air, and moreover it is susceptible to considerable contamination problems.
It has been also proposed to operate with heat bridges so as to insure a direct heat conduction from the heating plate to the endless pressing band through pressing shoes. This approach is disclosed in the German document DE-PS 33 25 578. The pressing shoes which provide the heat conduction have however considerable disadvantages. In addition to the very high force requirement for driving the pressing band, metal wear also occurs on the pressing shoes and therefore a reliable operation of the peripherally closed seals for the pressure chamber becomes questionable.
Finally, another system has been proposed, in which a pressure medium is no longer heated in the action zone but instead a steam (U.S. Pat. No. 2,135,763) or hot water (German document DE-OS 19 53 816) are supplied to the action zone in a circulation circuit. The heating is performed outside of the action zone, and the system operates with conventional supply speeds or withdrawal speeds for steam and hot water. These systems have to be considered as not suitable in praxis, since naturally the supplied hot pressure medium is cooled during its movement through the action zone. Since such action zones are relatively long, an unacceptable temperature drop in transporting direction of the workpiece is produced from the inlet to the outlet of the arrangement.